Ferromagnetic speed sensor

ABSTRACT

In order to ensure an optimal position of a magnet (3) in relation to a Hall IC (2) or, respectively, its Hall effect elements (4, 5), the magnet (3) and the Hall IC (2) are displaceable in relation to one another and, namely, in the direction of the lateral distance of the two Hall effect elements (4, 5). It is preferred that the Hall IC (2) is retained stationarily in the rotational speed sensor, and the magnet (3) is retained so as to be displaceable and lockable in position, in particular in a base member (10) of the sensor housing.

The present invention relates to a rotational speed sensor, inparticular a gear wheel sensor, including a magnet and two Hall effectelements cooperating therewith and arranged in laterally spacedrelationship one to the other, wherein a component part made offerromagnetic material, in particular a gear wheel, which has anirregular periphery or a periphery with points of discontinuity, and theangle of rotation or rotational speed of which is to be determined, ismovable past the Hall effect elements in the direction of the lateraldistance.

The rotational speed of a component part or only its special position onstandstill can be determined by means of such a rotational speed sensorand can be converted into a corresponding signal which can be processedfurther in a corresponding electronic device. Rotational speed sensorsof this type are used whereever it is intended to sense rotationalspeeds or only angles of rotation. A special field of application areautomotive vehicles and, more particularly, the automatic brake system(ABS) or the traction slip control (TSC) for driven wheels, or evenengine management and gear management.

Such rotational speed sensors are high-precision apparatus, the elementsof which need not only be manufactured very accurately, but also must beassigned one to the other. This applies in particular to the magnets andthe two Hall effect elements.

In the case of a gear wheel sensor, the teeth of the gear wheel oneafter the other move past the Hall IC with the two Hall effect elements,the magnets and the electronic devices, such as a protective wiring, forexample. Each tooth produces a pulse, and the rotational speed can bedetermined by computing the pulses. However, to be more precise, thematter is that the rotational speed sensor does not recognize the toothas such, but recognizes each respective transition from tooth to toothspace, or vice versa. Thus, a complete signal results when a tooth and atooth space each have moved past the rotational speed sensor or, inother words, when the gear wheel has been rotated further by one unitconsisting of tooth and tooth space.

First of all, the sensor senses the so-called basic field of the magnet.Added to the field intensity of the basic field is still a field densitywhich originates from the mass of the gear wheel or any similarcomponent part. If, instead of a tooth space, a tooth is assigned to thesensor, this results in a repeated amplification of the magnetic fieldfor the time the tooth is assigned to the sensor. Thus, when the gearwheel rotates, the field intensity varies in dependence on the teeth andtooth spaces moving past the sensor.

However, as the two Hall effect elements of the rotational speed sensorare offset in the circumferential direction of the gear wheel or thecomponent part, respectively, when viewed in the direction of rotationof the gear wheel, the tooth first arrives at the first Hall effectelement and, after a certain angle of rotation, at the second one. Theresultant oscillations of the magnetic field of the two sensors aredeferred in relation to each other.

If, however, a stationary tooth is simultaneously opposite to both Halleffect elements, the total magnet intensity at both Hall effect elementsis of equal size. When forming the difference in a correspondingsum-and-difference amplifier, the value zero results. As stated before,when the gear wheel or any component part of the like rotates, the twooscillations are deferred in relation to each other, asum-and-difference amplifier issuing a finite signal as a result.However, the values for the basic field and the mass of the componentpart neutralize each other so that the oscillation comprises only theincrease in the magnetic field density which is due to the tooth.

The analog signal, which is furnished to the two Hall effect elements ina deferred manner, as stated above, and which a sum-and-differenceamplifier emits, can be converted into a digital pulse train by means ofa so-called Schmitt trigger. The pulse train can then be delivered to acorresponding electronics for processing.

It becomes apparent from the previous description that the differencesignal corresponds to the theoretical value only if all conditions arecorrectly fulfilled. However, this is not true in practice because themagnets used happen to differ considerably from the ideal value inrespect of their field intensity. With respect to the surface of e.g.the effective pole, the variation of the field intensity can differ suchthat, in spite of a correct geometrical assignment of the magnet to thetwo Hall effect elements, the Hall effect elements sense differingvalues of the magnetic field intensity of the basic field. However, asthe magnets usually are permanent magnets and, consequently, theirstructure may be faulty, this problem cannot be eliminated in prior-artrotational speed sensors. Things are similar as regards themanufacturing tolerances of the analyzing circuit. It cannot be avoidedthat there are asymmetries between the Hall effect elements or in thedifference signal. The result is an impaired operation of the rotationalspeed sensor up to a complete malfunction.

Consequently, the object is to develop a rotational speed sensor of thetype previously referred to such that defects in the structure of themagnet or the analyzing circuit or the elements, respectively, can besensed and compensated in order to thereby ensure the proper operationof the rotational speed sensor.

To achieve this object, it is proposed by the present invention that therotational speed sensor according to the preamble of claim 1 isconfigured according to the characterizing portion of this claim.

In case the magnet or the analyzing circuit or the elements,respectively, exhibits a defective structure which has as a result, forexample, an asymmetrical variation of the magnetic field intensity overthe surface or only errors in the curve course, which leads to differentoutput signals of the two Hall effect elements, while the rotationalspeed sensor, geometrically, is aligned correctly in relation to thecomponent part or a tooth of the gear wheel, this can be corrected bydisplacing the magnet within the rotational speed sensor in thedirection of the distance of the two Hall effect elements so long untilboth, with the tooth stationary, determine the same total intensity ofthe magnetic field. Now, a correct difference signal can be formed alsoby way of a sum-and-difference amplifier, which can then be analyzed or,in case of need, can be converted and analyzed. The total fieldintensity of the two Hall effect elements can be easily displayed,monitored and corrected by means of appropriate recording devices.

Thus, the optimal mounting position of the magnet in relation to theHall IC can be determined accurately in a simple way.

According to an improvement upon the present invention, the Hall IC isrigidly mounted to the rotational speed sensor, while the magnet issupported so as to be slidable relative to the Hall IC. Compared to adisplacement of the Hall IC, this is the more favourable solutionbecause the magnet is not provided with electrical connections.

A particularly preferred embodiment of the present invention ischaracterized by the magnet being secured in a holding fixture and theholding fixture being slidably retained in a housing of the rotationalspeed sensor. This permits a conventional form of the magnet despite itsslidable mounting support. It is preferred that the magnet has acircular cylindrical configuration.

Another variant of the present invention is characterized by alongitudinal guidance designed in the type of a groove-and-tongue joint,the tongue being, preferably, provided on the holding fixture of themagnet. The holding fixture can be made easily from sheet metal or thelike, however, it must not impair the magnetic properties.

Further, it is very favourable that the magnet holding fixture is fixedafter the adjustment of the rotational speed sensor, in particular bycaulking. Especially in the last mentioned case, the optimal positionfound can be secured by minute means.

Hereinbelow, the present invention will be explained in more detail withreference to the accompanying drawings. The drawings show differentembodiments of the invention. In the drawings,

FIG. 1 is a side view of a first embodiment, the cross-section takenpartly in longitudinal direction;

FIG. 2 is a side view of a second embodiment of the invention, the gearwheel being outlined;

FIG. 3 is a top view of FIG. 2 along with a gear wheel;

FIG. 4 is a perspective and explosion view of a third variant of thepresent invention.

It shall be assumed in the following that in all cases a gear wheelsensor is referred to which is to be used in conjunction with a gearwheel 1. The area which includes the Hall IC 2 and the magnet 3 isassigned to the teeth 6 of the gear wheel 1 in the manner to be seen inFIGS. 2 and 3. The Hall IC 2 is equipped with two Hall effect elements 4and 5 which, according to FIG. 3, are offset laterally one to the other,when viewed roughly in circumferential direction of the gear wheel 1, sothat each tooth 6 at first reaches the Hall element 5 and, subsequently,reaches the Hall element 4 when the gear wheel rotates in the directionof the arrow 7.

According to FIG. 4, each sensor is equipped with a plate 8incorporating a series of electronic structural elements, in particulara protective wiring. In addition, there are inner electrical connectingelements 9, a base member 10, two covers 11, 12--in the embodiment ofFIG. 4--as well as a connecting cable 13. Further elements (notdescribed in more detail) can be of conventional type.

It can be seen in FIGS. 2 and 3 that there is a predefined gap distance14 between the gear wheel 1 and the sensor in the area of the Hall IC 2and the magnet 3, and that the two Hall effect elements 4 and 5 areassigned to a tooth, which is disposed on the Y-axis 16, for example, insuch a way that they have the same lateral distance from this Y-axis,when viewed in the direction of rotation or sidewise.

If the magnet 3 is not free from defects, or the Hall effect elements 4and 5 or, respectively, the analyzing circuit thereof, electrically, arenot of a completely symmetrical design, which occurs frequently inpractice, this means that in the configuration according to FIG. 3 theHall effect elements 4 and 5 detect a different total magnetic fieldalthough, theoretically, it would have to be precisely the same. Aboveall, the reason for this is that the magnetic field extendsasymmetrically, for example with respect to the Y-axis 16, or one of theelements exhibits a greater susceptibility. This may have as an effectthat, for example, the field intensity from the magnet results in agreater value at Hall effect element 5 than at Hall effect element 4, orvice-versa.

In order to permit a basic adjustment which corrects such "errors" ofthe magnet, it is suggested according to the present invention that themagnet 3 according to FIG. 4 is inserted into a holding fixture 16 whichis slidable in the sense of the double arrow 17 in the interior of therotational speed sensor and, respectively, in relation to the basemember 10, and which is lockably held in the final displacementposition. The holding fixture of a circular cylindrical magnet 3, forexample, can consist of two arcuate holding lugs which areinterconnected through a bottom bridge 18 of the holding fixture 16. Atthe side of the bottom bridge 18 pointing away from the magnet 3, atongue 19 is provided which, in conjunction with a groove 20 of the basemember 10, forms a groove-and-tongue joint which permits properdisplacement of the holding fixture 16 and, thus, also of the magnet 3in the direction of the double arrow 17, that means in the direction ofthe lateral distance of the two Hall effect elements 4 and 5. For thesake of clarity, the double arrow 17 is sketched once more in FIG. 3.Also, the groove-and-tongue joint 19, 20 is outlined in FIG. 1.Consequently, the displacement is performed perpendicular to the drawingplane in FIG. 1. After the adjustment of the magnet in relation to theHall IC 2 and, respectively, its two Hall effect elements 4 and 5, theholding fixture 16 in the base member 10 is locked in position bycaulking. Moreover, it can be seen in FIG. 4 that the holding fixture 16can be displaced, for example, in a tunnel-shaped seating 21 of the basemember 10, whereby it is guided in an anti-lift manner.

The mode of operation of this rotational speed sensor is known and,therefore, need not be explained in more detail. Whenever a signalperiod consisting of tooth and tooth space is terminated, the Hall IC 2each time issues a pulse to a control system, for example, for ABS, TSCor engine management of an automotive vehicle. The rotational speed ofthe gear wheel 1 and of elements rotatably connected therewith can bejudged from the number of teeth and the number of pulses.

We claim:
 1. A rotational speed sensor, in particular, a gear wheelsensor, comprising:a magnet and two Hall effect elements cooperatingtherewith, each Hall effect element arranged along an axis in alaterally spaced relationship one to the other, said magnet producing amagnetic field; means for displacing said magnet only in a directionthat is parallel to said axis and relative to said Hall effect elements,wherein a displacement of said magnet compensates for incorrect functionof said Hall effect elements caused by nonuniformities in said magneticfield, wherein said Hall effect elements are rigidly mounted in therotational speed sensor, and said magnet is supported so as to beslidable relative to said Hall effect elements, wherein said magnet issecured in a holding fixture, and the holding fixture is supported andis slidable in a housing of the rotational speed sensor, wherein saidholding fixture includes longitudinal guidance which is provided in thetype of a groove-and-tongue joint, said tongue arranged on the holdingfixture of the magnet.
 2. A rotational speed sensor as claimed in claim1, wherein said Hall effect elements are rigidly mounted in therotational speed sensor, and said magnet is supported so as to beslidable relative to said Hall effect elements.
 3. A rotational speedsensor as claimed in claim 2, wherein said magnet is secured in aholding fixture, and the holding fixture is supported so as to beslidable in a housing of the rotational speed sensor.
 4. A rotationalspeed sensor as claimed in claim 3, wherein said holding fixtureincludes longitudinal guidance which is provided in the type of agroove-and-tongue joint, the tongue being preferably arranged on theholding fixture of the magnet.
 5. A rotational speed sensor as claimedin claim 3, wherein said magnet holding fixture is adapted to beingfixed relative to said Hall effect elements.